Within the field of optical fiber telecommunications, the current upper limit on the bit rate of detection arises from the birefringence distributed along the length of a single-mode optical fiber. The birefringence may be due to non-circularity of the core of the optical fiber, and to stresses within the fiber. The birefringence may also vary along the length of the fiber. However, the birefringence may also be caused by external forces acting upon the fiber, and to temperature variations, and these effects vary both along the length of the fiber and with time. The overall birefringence of a length of fiber thus varies over time with a magnitude which is random.
An ideal single-mode optical fiber guides optical power in the fundamental mode as two identical but orthogonal polarization modes, so that the modes are completely interchangeable. However, imperfections in the fiber and the effect of external parameters lead to the optical power within the two polarization modes both differing in magnitude and travelling at slightly different speeds so that a differential group delay exists between the modes. The birefringence determines the magnitude of the differential group delay, and the optical power within the two polarization, and so the width of an optical pulse travelling along the optical fiber will vary randomly by an amount determined by the random fluctuations in the birefringence. This effect is called the polarization mode dispersion, and it is of particular concern because it limits the performance of optical fiber telecommunications systems in a way that cannot be predicted accurately, and hence cannot be compensated.
Although polarization mode dispersion is usually considered to be a characteristic of the total length of an optical fiber, the effects which give rise to it may act over relatively short sections of the fiber. In particular, the polarization mode dispersion of an optical fiber cable may increase significantly after installation, arising from a change in the birefringence over one particular short length within the fiber. Accordingly, it would be useful to be able to measure characteristics related to the two polarization modes of a single-mode optical fiber, such as polarization mode dispersion, at different positions along the length of the optical fiber so that any local effect can be identified at a particular length position in the optical fiber. This would be particularly useful if the measurement were able to be made with access to just one end of the fiber in the same way that optical time domain reflectometers are used to measure the optical loss of fibers. Existing commercial apparatus can measure the overall polarization mode dispersion of an optical fiber and requires access to both ends of the fiber. The existing commercial apparatus does not enable the measurement of the polarization mode dispersion at different positions along the length of the optical fiber. The magnitude of the birefringence as it varies along the fiber may be characterized as a beat length, and the statistical correlation between two sections of fiber may be related by a correlation length. Both these parameters are useful for describing the behavior of the fiber, and the environment it is experiencing, and are inherently characteristics of length position within the fiber.
There are a number of known methods of measuring polarization mode dispersion, and associated characteristics of single-mode optical fibers, which provide a single measurement for the total length requiring access to both ends of the fiber. In addition, optical time domain reflectometry is a well-established technique for measuring the optical loss of an optical fiber at different positions along the length of the fiber and requiring access to only one end of the fiber. The present invention is based on the discovery that it is possible to apply the existing measurement techniques of polarization mode dispersion to modified versions of optical time domain reflectometry apparatus, and thus to derive useful measurements.